IK Arm Rigging with more than 3 bones
So if you're trying to limit the rotation between the 2 lowerarm bones, what you're looking for is probably the IK Limits: In Pose mode, select the bone you want to constrain, go in the Properties panel &gt; Bone &gt; Inverse Kinematics (this menu appears as soon as the bone is influenced by an IK), then limit on the axis you want, in that case X and Z.
As you mentioned, in 2.8 you can simply click on the Lock IK option (top of the same menu).